The present invention relates generally to the field of shielded ribbon cables and more particularly to mass terminable shielded ribbon cables exhibiting desirable electrical characteristics.
There exists a need for an electrical signal transmission cable which has both desirable signal transmission line characteristics and desirable physical characteristics. In order to exhibit desirable signal transmission line characteristics, the particular cable must exhibit low distortion, low attenuation at high frequency, radiate little electro-magnetic interference, not be susceptible to electro-magnetic interference, and exhibit a low amount of crosstalk between signal conductors, forward and backward. Desirable physical characteristics in a cable are the use of a multiplicity of signal conductors, capability for easy mass termination, low cost, flexibility and compactness.
There exists in the market place a multiconductor, flexible, mass terminable ribbon cable such as the 3365 cable manufactured by Minnesota Mining and Manufacturing Company, St. Paul, Minn. and sold under the trademark Scotchflex. While this is a very useful product, there are a number of uses of ribbon cable where the electrical characteristics of this cable are not sufficient. Such applications may involve the connection of a digital computer to a remote peripheral unit, such as a disk storage unit, printer, keyboard, display or modem. In these situations, it may be desirable and necessary to utilize a cable which exhibits desirable signal transmission characteristics.
Some critical cable applications requiring signal transmission line characteristics have been met with coaxial cables. With coaxial cables individual signal conductors are encased in individual shields. While exhibiting desirable electrical signal transmission line characteristics, these cables, however, suffer the disadvantage of the lack of a multiplicity of conductors and the lack of easy mass termination as well as relatively high initial cost.
One type of prior art cable is a cable known as a ribbon coaxial cable. In a ribbon coaxial cable, a plurality of separate coaxial cables are packaged together to form a ribbon cable. Each individual signal conductor is wrapped with its own separate individual shield. An example of this type of cable is Underwriters Laboratory (UL) Style No. 2741 cable. While this type of cable does provide generally good transmission line electrical characteristics, it suffers from many disadvantages. A typical example of this product contains signal conductors on 100 mil (2.54 millimeters) centers as opposed to the more typical 50 mil (1.27 millimeters) centers with the previously mentioned ribbon cable. The ribbon coaxial cable is not as compact, of course, because of the necessity of wrapping each individual signal conductor with its individual shield. In addition to being relatively expensive to manufacture, the ribbon coaxial cable is bulky due to the spacing of the individual signal conductors and, in addition, is not easily mass terminable. Since each individual signal conductor carries its own shield, the termination process involves separately stripping and terminating each individual shield wire, hardly a mass termination operation. Further, the particular UL Style 2741 cable uses a helical wrap of a thin polyester film/aluminum foil laminate as its shield which does not necessarily provide good electrical continuity. In order to help correct this problem, the 2741 cable uses a drain wire run longitudinally along the cable with the shield to attempt to provide good longitudinal electrical continuity. However, since the drain wire is not connected to the shield but makes intermittent and variable contact with the shield, the electrical characteristics of the cable are not uniform along its length and tend to vary from signal conductor to signal conductor. These variable electrical characteristics results in a skewing of electrical pulses simultaneously applied to more than one signal conductor and to higher attenuation of the electrical pulses than occurs with a longitudinally continuous shield.
Historically, a shielded cable has meant any of a variety of cables which include a cable with a shield only on one side of the ribbon cable or even in some instances a shield on both sides of the ribbon cable but without shielding along the cable edges or without electrical continuity between the shield on each side. In order to eliminate electro-magnetic interference, both radiation and susceptibility, it is necessary to have a full 360 degree transverse shield around the ribbon cable. Thus, for purposes of this invention, a shielded cable means a cable which is fully shielded with a 360 degree circumferential transverse shield providing full electrically continuity, both transversely and longitudinally. A ribbon cable with a shield on one side only or a ribbon cable with a shield along both sides without shielded edges is not a true shielded cable and will not prevent electro-magnetic interference.
There are several examples of prior art ribbon cables which utilize conductive shielding on only one side. These cables suffer adverse electrical characteristics with increased signal attenuation over a comparable cable without shield and an increased rise time degradation. Further, the one side shield will not provide full shielding against electro-magnetic interference. U.S. Pat. No. 4,209,215, Verma, Mass Terminable Shielded Flat Flexible Cable and Method of Making Such Cables, provides a typical ribbon cable with a one-side shield. This cable, however, does not provide desirable electro-magnetic interference protection. U.S. Pat. Nos. 3,576,723, Angele et al, Method of Making Shielded Flat Cable, and U.S. Pat. No. 3,612,743, Angele et al, Shielded Flat Cable, provide a ribbon cable coated with a shielding material on one side. Again, this cable suffers disadvantages because it is only a single-sided shield. U.S. Pat. No. 3,818,117, Reyner II, Low Attenuation Flat Flexible Cable, is another typical single-sided shield cable. However, the Reyner cable is not even a good single-sided shielded cable because the conductive ground plane contains slots which are used to control the impedance and cable attenuation characteristics.
Some prior art cables utilize a double side shield but without full 360 degree shielding. U.S. Pat. No. 3,757,029, Marshall, Shielded Flat Cable, is a typical example of a ribbon cable with a double side shield. However, notice that in Marshall, the shield is not a full 360 degree transverse shield as the sides of the ribbon cable are open and are not shielded. Further, the conductive metallic strips used to provide the shield on both sides do not provide electrical continuity with each other. This cable suffers from inadequate protection from electro-magnetic interference and from a non-uniform characteristic impedance because of the lack of bonding of the shield to the cable dielectric, and also has electrical characteristics which are not suitable for fast rise time transmission line cable. U.S. Pat. No. 3,700,825, Taplin et al, Circuit Interconnecting Cables and Methods of Making Such Cables, is another example of a cable with a double side shield. An open lattice structure is used on both sides of the cable. However, the lattice structures on opposite sides are not interconnected and this cable does not provide a full 360 degree shield. U.S. Pat. No. 3,612,744, Thomas, Flexible Flat Conductor Cable of Variable Electrical Characteristics, also shows a cabe with a double sided shield. Perforated foil is utilized with a longitudinal drain wire on each side along with several separate distinctive dielectric layers. Again the ground planes provided by the perforated foil and the drain lines are not interconnected and do not provide a full 360 degree shield. All of these cables suffer from inadequate protection from electro-magnetic interference.
Some prior art cables have utilized a full 360 degree transverse shield but suffer in their electrical characteristics. U.S. Pat. No. 3,634,782, Marshall, Coaxial Flat Cable, provides a ribbon cable which has a 360 degree transverse shielded braid. While this cable does have a full shield against electro-magnetic interference, it suffers from other disadvantages. The shielded braid is not necessarily bonded to the cable dielectric. This lack of bonding will provide a non-uniform dielectric constant, both transversely and longitudinally from conductor to shield. This will result in excessive forward crosstalk and will result in non-uniform characteristic impedance. Another cable having a full 360 degree shield a vinyl insulated ribbon cable with a vinyl jacket covering the loose electro-magnetic shield such as the 3517 cable manufactured by Minnesota Mining and Manufacturing Company, St. Paul, Minn. and sold under the trademark Scotchflex.RTM.. While this cable provides for adequate protection against electro-magnetic interference, the use of the vinyl insulation and the lack of bonding of the shield to the insulation and lack of other geometric considerations provide electrical characteristics which are not suitable for high speed data transmission line applications. Another example of a ribbon cable attempting to be both shielded and have desirable electrical characteristics is a cable which is manufactured by Spectrastrip, 7100 Lampson Avenue, Garden Grove, Calif. The cable construction is a standard 60 conductor, 28 American Wire Gauge stranded copper with gray vinyl insulation in a double hump profile with the cable 36 mils (0.91 millimeters) thick at the humps. A shield is provided on both sides using two layers of an aluminum foil and polyester film construction similar to the Sun Chemical 1001 film with the foil sides of both layers facing the same direction so that they overlap at the edge and provide electrical continuity. A heavy black vinyl jacket is extruded over the shield. On one side of the cable the jacket forces the shield layer which has the polyester side toward the signal conductors to conform to and adhere to the vinyl. On the opposite side of the cable the polyester side of the shield layer bonds to the jacket leaving a variable air gap between the aluminum and the insulation containing the conductors. This cable shows a variable characteristic impedance and an excessive voltage attenuation, along with excessive rise time degradation. U.S. Pat. No. 3,582,532, Plummer, Shielded Jacket Assembly for Flat Cables, shows a zipper jacketed shielded cable. The shield is attached to the interior of the jacket. The variable spacing between the shield and the insulation results in a variable charactistic impedance and unpredictable crosstalk.
Some prior art cables have utilized a plurality of layers of differing dielectrics to reduce forward crosstalk. U.S. Pat. No. 3,763,306, Marshall, Flat Multi-Signal Transmission Line Cable With Plural Insulation, provides a ribbon cable with this construction. This cable is a ribbon cable with a multiplicity of signal conductors but with two distinctly different dielectrics around the signal conductors. The cable has a jacket encasing a standard insulation with a material of a higher dielectric constant than the standard dielectric. This cable is not shielded and also suffers the disadvantage of exhibiting excessive backward crosstalk. U.S. Pat. No. 3,735,022, Estep, Interference Controlled Communications Cable, also illustrates an attempt to control crosstalk by providing a cable with dual differing dielectric materials.
These prior art cables demonstrate that many attempts have been made to achieve a shielded, mass terminable, multiple conductor, flexible ribbon cable having electrical characteristics suitable for transmission line characteristics. These prior art cables also demonstrate that the prior attempts at a total solution to this problem have failed. These prior art cables demonstrate the complexity of cable construction having suitable transmission line electrical characteristics and demonstrate that it is not possible to simply wrap a metal shield around an existing flexible ribbon cable and achieve suitable electrical transmission line characteristics. The problem is complex, and the results achieved depend upon many interrelated physical characteristics.